This invention relates to the binary modulation of injection lasers, and is particularly concerned with the regulation of the bias level and modulation depth to provide operation which on the one hand provides good extinction, while on the other hand does not produce excessive chirp. Chirp needs to be limited, particularly in a long distance transmission system, so as to avoid imposing unnecessary bandwidth limitations as a result of the pulse-broadening effects of chromatic dispersion exhibited by that system. Chirp can be limited by ensuring that the drive maintains the laser somewhat above its lasing threshold even during the lower drive current (data 0) bit periods, but the higher that the current is maintained for data 0 bit periods, the poorer the extinction.
A difficulty with trying to achieve optimal drive conditions is that the lasing threshold current, and the slope of the laser characteristic which plots light output as a function of current drive, are liable to vary not only from laser to laser, but also, in any given laser, as a function of its temperature and as a property of the ageing process of that laser.
One way of dealing with these problems is to employ two feedback control loops. One of these loops derives a feedback control signal that is indicative of the mean light output power from the laser and employs this to regulate the laser. Assuming the use of a balanced code, and a linear relationship between light output and laser drive between the data 0 drive current level i.sub.0 and the data 1 drive current level i.sub.1, the loop operates to provide a mean drive current (i.sub.0 +i.sub.1)/2. The other feedback loop derives a feedback control signal that is indicative of the lasing threshold and employs this to regulate the value i.sub.0 to be just above the lasing threshold value. If it is safe to assume that the slope of the laser characteristic between current values i.sub.0 and i.sub.1 will not vary too much, the first loop can be dispensed with. Under these circumstances the sole feedback control loop regulates the laser bias and the modulation depth (i.sub.1 -i.sub.0) is maintained at a constant preset value.
One particular way of constructing a feedback control loop to derive a feedback control signal indicative of the laser threshold current is described by D. W. Smith et al in the paper entitled `Laser Level Control for High Bit Rate Optical Fibre Systems` given at the 13th Circuits and Systems International Symposium Houston April 1980. This involves impressing a small amplitude low frequency current modulation, a ripple, upon the data `0`s, this low frequency modulation being within the pass-band of a monitor photodiode and its associated circuitry. The slope of the characteristic which plots light output as a function of drive current, changes as the drive current is increased through the lasing threshold from a relatively lower value to a relatively higher value. The ripple on the laser drive current therefore produces a corresponding ripple on the monitor photodiode current, and this is compared with a preset value to provide a feedback control signal which is employed to regulate I.sub.0 to a particular slope value intended to correspond to a point just above lasing threshold. Problems encountered with this approach are attributable to the fact that the shape of the characteristic is liable to vary from laser to laser, making it difficult to choose an optimum value of slope as the target of the feedback loop, and also to the fact that an individual laser may exhibit that target value of slope over an extended range of current drive levels in the vicinity of the optimum operating point.